1. Mendel's Laws of Inheritance
| Law | Statement | Monohybrid cross evidence |
|---|---|---|
| Dominance | In a heterozygote, one factor expresses itself while the other is masked. | All F1 plants were tall after crossing true-breeding tall and dwarf pea plants. |
| Segregation | The two alleles of a gene separate during gamete formation and remain uncontaminated. | F2 phenotypic ratio 3 tall : 1 dwarf; genotypic ratio 1 TT : 2 Tt : 1 tt. |
| Independent assortment | Alleles of different genes assort independently when they are unlinked. | Dihybrid F2 ratio 9:3:3:1 in Mendel's seed-shape/seed-colour cross. |
2. Incomplete Dominance vs Codominance
| Feature | Incomplete dominance | Codominance |
|---|---|---|
| Heterozygote phenotype | Intermediate between two homozygotes | Both alleles express fully and simultaneously |
| Example | Snapdragon: red × white gives pink F1 | AB blood group: IA and IB both express |
| F2 phenotypic ratio | 1 red : 2 pink : 1 white; phenotypic and genotypic ratios are same | Depends on allelic combination; AB phenotype is distinct |
| Key distinction | No allele is completely dominant, but phenotype is blended/intermediate | Not blending: both gene products are detectable |
3. ABO Blood Groups
| Blood group | Genotype(s) | Antigen on RBC | Antibody in plasma |
|---|---|---|---|
| A | IAIA or IAi | A | Anti-B |
| B | IBIB or IBi | B | Anti-A |
| AB | IAIB | A and B | None |
| O | ii | None | Anti-A and Anti-B |
NCERT cueIA and IB are codominant; both are dominant over i. The population has three alleles, though each individual carries only two.
4. Linkage vs Recombination
| Point | Linkage | Recombination |
|---|---|---|
| Meaning | Genes on same chromosome tend to be inherited together | New combinations of parental characters are produced |
| Cause | Physical proximity of genes on a chromosome | Crossing over between homologous chromosomes during meiosis |
| Relationship | Stronger linkage gives lower recombination frequency | Higher frequency indicates greater distance between genes |
| Application | Explains deviation from independent assortment | Used by Morgan to construct chromosome maps |
5. Sex Determination Systems
| Organism / system | Female | Male | Who determines sex? |
|---|---|---|---|
| Human / XX-XY | XX; all ova carry X | XY; sperm carry X or Y in equal proportion | Male; X sperm gives XX female, Y sperm gives XY male |
| Bird / ZZ-ZW | ZW | ZZ | Female produces Z- and W-bearing eggs |
| Honey bee / haplodiploidy | Diploid from fertilised egg | Haploid from unfertilised egg | Fertilisation status determines sex |
6. Mendelian vs Chromosomal Disorders
| Category | Cause | Examples |
|---|---|---|
| Mendelian disorders | Alteration or mutation in a single gene; may be autosomal or sex-linked | Haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria, thalassaemia |
| Chromosomal disorders | Gain/loss/abnormal arrangement of chromosome(s), often due to nondisjunction | Down's syndrome, Klinefelter's syndrome, Turner's syndrome |
| Down's syndrome | Trisomy of chromosome 21 | 47 chromosomes; characteristic facial features and intellectual disability |
| Klinefelter's syndrome | XXY male | 47 chromosomes; sterile male with feminine development |
| Turner's syndrome | XO female | 45 chromosomes; sterile female with underdeveloped ovaries |